Nils Krogstad, the play's antagonist, uses some seriously villainous tactics over the course of the play. He intimidates, blackmails, and threatens Nora in an effort to keep his job at the bank. After Torvald fires him, Krogstad takes it a step further, saying,
"I want to get into the Bank again, in a higher position. […] Within a year […] It will be Nils Krogstad and not Torvald Helmer who manages the Bank." (2.83-2.285)
Now, if this were a melodrama, Krogstad would most likely twirl his black mustache and cackle diabolically after such a statement. This, however, is realism; Ibsen's characters demonstrate a nuanced complexity totally absent in the characters of the popular melodramas of his day.
Krogstad is a prime example of these new, more textured, more realistic characters. For starters, his goal isn't evil for evil's sake. He's not plotting world domination or anything. Krogstad just wants to regain his standing in the community. He tells Nora, "I want to rehabilitate myself" (2.83).
Ever since he got caught in a forgery scheme back in the day, everybody thinks he's a nasty, terrible person. Sure, he did commit a crime, but it was pretty small. Nora, our sympathetic protagonist, is guilty of the exact same thing. After the community turned its back on him, Krogstad was forced into the unsavory business of moneylending and blackmailing in order to support his family. In a way, it was the community's close-minded lack of forgiveness that created him. Here again we see the central motif of all of Ibsen's plays: the individual vs. society.
We get hints throughout the whole play that, underneath Krogstad's villainous exterior, there's a respectable gentleman waiting to emerge. Whenever he deals with Nora, he's pretty courteous (for a blackmailer). One of the most poignant moments between the two is when they commiserate about their suicidal thoughts. He tells her, "Most of us think of that at first. I did, too—but I hadn't the courage" (2.271). She replies quietly, "No more had I" (2.272).
When Krogstad reunites with Christine, he is fully redeemed. If not for Christine's dissuasion, he would've even demanded his letter back unopened, so that Torvald would've never known anything. Instead he writes a new letter, telling the Helmers that he "regrets and repents" his actions, and willingly releases them from his clutches (3.249).
It's interesting that our antagonist's final revelation is one of self-fulfillment, just like our protagonist Nora. Yes, it seems that when Christine reassuringly says, "Nils, I have faith in your real character" (3.58), Krogstad is finally able to once again find faith in himself.
Ugh. If only the skeezy villains on Game of Thrones could rehabilitate that easily!